Medical imaging technologies can provide detailed information useful for differentiating, diagnosing, or monitoring the condition, structure, and/or extent of various types of tissue within a patient's body. In general, medical imaging technologies detect and record manners in which tissues respond in the presence of applied signals and/or injected or ingested substances, and generate visual representations indicative of such responses.
A variety of medical imaging technologies exist, including Computed Tomography (CT), Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT), and Magnetic Resonance Imaging (MRI). Any given medical imaging technology may be particularly well suited for differentiating between specific types of tissues. A contrast agent administered to the patient may selectively enhance or affect the imaging properties of particular tissue types to facilitate improved tissue differentiation. For example, MRI may excel at distinguishing between various types of soft tissue, such as malignant and/or benign breast tumors or lesions that are contrast enhanced relative to healthy breast tissue in the presence of Gadolinium DPTA or another contrast agent.
Particular imaging techniques, for example, certain MRI techniques, may scan a volume of tissue within an anatomical region of interest. Scan data corresponding to an anatomical volume under consideration may be transformed into or reconstructed as a series of planar images or image “slices.” For example, data generated during a breast MRI scan may be reconstructed as a set of 40 or more individual image slices. Any given image slice comprises an array of volume elements or voxels, where each voxel corresponds to an imaging signal intensity within an incremental volume that may be defined in accordance with x, y, and z axes or dimensions. The z axis commonly corresponds to a distance increment between image slices, that is, image slice thickness.
Medical imaging techniques may generate or obtain imaging data corresponding to a given anatomical region at different times or sequentially through time to facilitate detection of changes within the anatomical region from one scan series to another. Temporally varying, tissue dependent contrast agent uptake properties may facilitate accurate identification of particular tissue types. For example, in breast tissue, healthy or normal tissue exhibits different contrast agent uptake behavior over time than tumorous tissue. Moreover, malignant lesions exhibit different contrast agent uptake behavior than benign lesions (“Measurement and visualization of physiological parameters in contrast-enhanced breast magnetic resonance imaging,” Paul A. Armitage et al., Medical Imaging Understanding and Analysis, July 2001, University of Birmingham).
In view of the foregoing, comparisons between 1) an image obtained prior to contrast agent administration (i.e., a “pre-contrast image”) and one or more corresponding images obtained following contrast agent administration (i.e., “post-contrast images”); and/or 2) a temporal sequence of post-contrast images relative to each other may serve to highlight differences between and/or within tissues, thereby aiding medical diagnostic procedures.
Medical images can be characterized by their spatial resolution. As previously indicated, an MRI slice comprises a set of volume elements or voxels, where each voxel corresponds to a signal intensity or value for a quantized tissue volume. An exemplary MRI slice may have a resolution of 256×256 voxels with respect to x and y reference directions or axes, where each voxel represents imaging data for a 1.0×1.0×2.5 mm3 tissue volume relative to x, y, and z axes, respectively.
Successful detection, characterization, and/or identification of tissue boundaries and/or small tissue structures such as newly or recently developed lesions or tissue abnormalities requires the ability to identify tissue boundaries and/or indicate temporal tissue changes at the level of fractional voxels, individual voxels, and/or very small voxel groups. If a patient moves even slightly during or between image acquisition procedures, the imaged shape, size, and/or relative location of a given tissue boundary or structure may be distorted or shifted relative to its actual shape, size, and/or location. Unfortunately, some patient movement will essentially always exist. Patient movement may arise from several sources, including changes in patient relaxation or tension levels over time, for example, prior to, during, and following injection of a contrast agent; minor positional adjustments; and respiration. Patient movement can be particularly problematic when imaging nonrigid or readily deformable anatomical structures such as breasts.
To reduce the effects of patient motion upon imaging accuracy, medical imaging techniques may include registration correction procedures. Current registration correction procedures involve selection of a reference image from within an image series; generation or determination of motion estimation parameters; and motion correction of acquired images with respect to the reference image. The motion correction involves image resampling with subvoxel accuracy. Such resampling may occur, for example, through an interpolation procedure. Unfortunately, image resampling itself can degrade or deteriorate the spatial resolution of imaging information. Such degradation can be dependent upon one or more aspects of the registration correction procedure itself. A need exists for a system and method that situationally consider the potential impact that registration correction may have upon imaging accuracy.